Geochemistry of gases in seismogenic depths of the San Andreas Fault Zone
Final Report Abstract
The SAFOD project was carried out to achieve a better understanding of the processes occurring in active plate-bounding fault systems at seismogenic depths. For this propose, two wells were drilled in 2002 (Pilot Hole, PH) and in 2004 and 2005 (Main Hole, MH-I & -II). The SAFOD well site is located 1.8 km southwest of the surface trace of the San Andreas Fault (SAF) in the Coast Range of central California, in the vicinity of Parkfield. The SAF separates the North American Plate in the northeast from the Pacific Plate in the southwest. In 2007, the presence of two active moving fault planes was finally proved, based on casing deformations in the MH in 3194 m and 3301 m depth. Consequently, sidetracks were drilled out of the MH in summer 2007 (SAFOD-III phase) to receive drill core samples from these sections of the SAF. During all drilling phases of SAFOD-III, real time gas monitoring was performed on returning drill mud. Noble gas isotope investigations as well as stable isotope investigations (δ13C, H/D) were performed on off-line gas samples. Due to the little yield of core material (only 37 m), the investigations on gas extraction and analysis of drill core samples could not be carried out as proposed. Five drill core samples were analyzed for their gas concentration and composition. Based on different gas and fluid compositions on the Pacific Plate and the North American Plate as well as different hydrological parameters, we concluded earlier that fluid migration transverse to the SAF is hampered by low permeable fault rocks. That means that the SAF acts as a fluid barrier, separating two distinct fluid regimes on the Pacific Plate and the North American Plate. Our new gas data from SAFOD-III studied in this project suggests that fluid migration barriers are associated with fault planes at 3194 m and 3301 m depth. These fault planes hamper advective fluid flow transverse to the fault and probably root deeper than the surrounding permeable hydrocarbon source or reservoir rock. Progressive lowering of C2H6/CH4, CJH8/CH4 and C4H10/CH4 ratios with increasing molecular mass below the fault plane at 3194 m, which again increase below the 3301 m fault plane, imply that hydrocarbon migration transverse to the fault planes is limited to diffusion processes that affect the molecular composition of hydrocarbons. The δ13C and H/D values of methane are quite homogenous through the investigated section (δ13C between -29.7 and -33.5%o PDB, H/D belween -173 and -154%o SMOW). The small variations do not show any trend with depth, which indicates a common near by source for hydrocarbons for the entire investigated section. The permeability along the fault (i.e. parallel to the fault direction) seems to be low in close vicinity to the fault core and possibly also in the fault core. In fact, the lowest 3He/4He ratios in the entire SAFOD wells were observed at 3144 m and 3313 m. However, our data imply that a fractured fault core halo, localized between the two creeping planes al 3194 m and 3301 m, builds up a conduit for mantle-derived fluids. Nevertheless, their principal path way seems to be permeable country rock in the North American Plate. It is, however, not clear weather the elevated 3He/4He values found between the active moving fault planes, which probably reflects fractured fault halo, are caused by temporal variation in the fluid flow due to fault valve behaviour, i.e. migration of fluids from greater depth after the 09/28/2004 earthquake, or continuous up-flow of mantle-derived fluids through the fault. Further investigations (e. g. regular downhole fluid sampling or continuous gas sampling and monitoring via U-tube or similar techniques) would be necessary to better understand this striking observation.
Publications
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(2007): Characterization of gas From Seismogenic Depths of the San Andreas Fault at SAFOD, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract H11B-0486
T. Wiersberg and J. Erzinger
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(2007): Hydrogen anomalies at seismogenic depths of the San Andreas Fault, Geochimica et Cosmochimica Acta, 71, 15, Suppl. 1, A1110, Abstracts of the 17th Annual V. M. Goldschmidt Conference Cologne
T. Wiersberg and J. Erzinger
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(2007): On the origin and spatial distribution of gas at seismogenic depths of the San Andreas Fault from drill mud gas analysis, IODP-ICDP Kolloquium 2007, Potsdam
T. Wiersberg and J. Erzinger
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2007. A helium isotope cross-section study through the San Andreas Fault at seismogenic depths, Geochem. Geophys. Geosyst., 8, Q01002
Wiersberg, T., and Erzinger, J.
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2007. Real-time mud gas monitoring: A technique to obtain information on the composition and distribution of gases at depth while drilling. Scientific Drilling, Special Issue I
Wiersberg T., and Erzinger J.
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(2008): Characterization of gas from seismogenic depths of the San Andreas Fault at SAFOD, IODP-ICDP Kolloquium 2008, Hannover
T. Wiersberg and J. Erzinger
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(2008): Characterization of gas from seismogenic depths of the San Andreas Fault at SAFOD, SDD01412L, 33rd International Geological Congress, Oslo
T. Wiersberg and J. Erzinger
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2008. On the origin and spatial distribution of gas at seismogenic depths of the San Andreas Fault from drill mud gas analysis. Applied Geochemistry 23, 1675-1690
Wiersberg T., and Erzinger J.
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(2009): Gas migration and permeability structure of the San Andreas Fault at SAFOD deduced from drill-mud gas monitoring data, IODP-ICDP Kolloquium 2009, Potsdam
T. Wiersberg and J. Erzinger
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(2009): The small-scale permeability structure of the San Andreas Faull at SAFOD deduced from drill mud gas data. Geophysical Research Abstracts, Vol. 11, EGU2009-6845, 2009. European Geosciences Union General Assembly, Vienna
T. Wiersberg and J. Erzinger
